WASHINGTON — President Barack Obama made a last-minute personal appeal to Democrats to pass landmark health care legislation today as the House opened debate on a bill to expand coverage to millions of uninsured Americans.

Emerging from a closed-door meeting with the president, House Speaker Nancy Pelosi predicted passage of the bill later in the day, adding, “We will pass health care reform.”

“He came here to say, ‘This is what we said we would do in the campaign. Let’s do it,'” Majority Leader Steny Hoyer said after the meeting.

The bill would cost $1.2 trillion over the next decade. It would provide health coverage to tens of millions of Americans who do not have it now, require most employers to offer it to their workers and prohibit insurance companies from denying coverage based on a person’s medical history.

“The status quo is unaffordable and unsustainable. Health care reform benefits all of us,” said Rep. Allyson Schwartz, a Pennsylvania Democrat, as debate opened on the House floor.

House passage of the bill is crucial if Obama is to accomplish his top domestic priorty and fulfill the biggest promise of his campaign last year. But the legislation still faces multiple hurdles and a Senate vote on it might not occur until next year.

Republicans were united in their opposition to the bill.

“The American people need to understand this is about a government takeover of the whole health care system,” said Rep. Paul Broun, a Georgia Republican.

The most contentious issue is a new government-run insurance plan that would be offered alongside private coverage within new purchasing marketplaces, or “exchanges,” where individuals and small businesses could shop for and compare options.

Obama made his trip to the Capitol complex as abortion rights lawmakers voiced anger at a last-minute concession granted by House Democratic leaders to foes of the procedure, who were given a vote on their proposal for stronger restrictions on abortion coverage.

“There is a risk” that some in the Pro-Choice Caucus would vote against the legislation if the stricter curbs are adopted, said Rep. Diana DeGette, a Colorado Democrat.

The abortion agreement was reached at midnight Friday after hours of intense negotiations brokered by House Speaker Nancy Pelosi. Anti-abortion Democratic lawmakers fought for and won an opportunity to insert tougher restrictions into the legislation during Saturday’s debate, despite fervent opposition from pro-choice liberals who are a driving force behind the overall bill.

“We wish to maintain current law, which says no public funding for abortion,” Democratic Rep. Bart Stupak of Michigan said. “We are not writing a new federal abortion policy.”

Federal law currently prohibits the use of federal funds to pay for abortions except in cases of rape, incest or situations in which the life of the mother is in danger. Left unresolved is whether individuals would be permitted to use their own funds to buy insurance coverage for the procedure in the federally backed insurance exchange envisioned under the legislation.

Stupak’s amendment would deny abortion coverage to anyone who gets federal insurance subsidies or buys a policy from the government. People could buy separate policies covering just abortions using their own money.

DeGette called Stupak’s amendment “the biggest restriction on a women’s right to choose that’s been considered on the floor of the House” in her 13 years in office.

The leadership’s hope is that no matter how the vote on the abortion measure turns out, Democrats on both sides of the abortion divide will then unite to give the health care bill a majority over unanimous Republican opposition.

With Democrats’ command of the necessary votes looking tenuous, Obama threw the weight of his administration behind the effort to round up support. He and top administration officials worked the phones to pressure wavering lawmakers.

Their message: “This is a historic moment. You don’t want to end up with nothing,” said Altmire, who remained undecided.

Democratic leaders hoped to hold the vote Saturday evening, but Majority Leader Steny Hoyer of Maryland said it could be delayed.

Democrats hold 258 seats in the House and can afford 40 defections and still wind up with 218, a majority if all lawmakers vote. But all 177 Republicans were expected to vote “no,” and Democratic leaders faced a series of complications trying to win the needed votes for their complex and controversial legislation that would affect one-sixth of the economy and touch the lives of countless Americans.

In the Republican Party’s weekly radio address, Mississippi Gov. Haley Barbour said Democrats should scrap their ambitious legislation and concentrate on modest health care changes that could find bipartisan support.

“The House Democrats’ health care bill should be withdrawn and reworked,” he said.

Tuesday’s elections — in which Democrats lost two governors’ races — sent a message that voters care about jobs, not growing the size of government, Barbour said.

Obama’s push to reform the health care system has already moved further along the legislative process than a similar effort by President Bill Clinton in his first-term in the 1990s. Democrats are mindful that the party lost control of Congress in the 1994 mid-term elections in part because of their failure to enact health care reform legislation.

Even if the House passes a health care bill, it by no means guarantees that any reform measure will reach the president’s desk to sign.

House rules make it easier for the large Democratic majority to advance legislation. But it will be tougher to get Senate approval because Democrats will need 60 out of 100 votes to end debate and bring legislation to a final vote, and several moderate Democratic senators still have reservations. If the Senate does pass a bill, it would have to be reconciled with the House version by a panel of lawmakers from both chambers before the legislation is put up for final approval.

The United States is the only developed nation that does not have a comprehensive national health care plan for all its citizens. The government provides coverage for the poor, elderly and military veterans, but most Americans rely on private insurance, usually provided through their employers.

But with unemployment climbing, many Americans are losing their health insurance when they lose their jobs. At the same time, the deepening budget deficit has made it difficult for lawmakers to support costly new programs.